The present invention relates to dispersions or slurries of particulate solids in water; in particular to dispersions of mineral solids which normally tend to aggregate, settle or phase out, and to a method of inhibiting those phenomena in slurries containing such mineral particles so as to improve the viscosity of such slurries making them more workable.
The present invention is suitable to produce slurries of various particulate solids, for example calcium carbonate for the paper industry and clays for use in the ceramic industry, but in fact the invention may be used for producing a workable aqueous dispersion or slurry of any solid particles in high a concentration.
Previous attempts at producing such aqueous dispersions are described in, for example, EP-B-0129329. This document discloses pigment slurries containing water soluble polymers of molecular weight from 1,500 to 6,000 formed from one or more ethylenically unsaturated monomers and containing acid groups selected from carboxyl and sulphonic groups. The present invention shows enhanced performance over this prior art.
U.S. Pat. No. 4,868,228 discloses grinding agents in aqueous suspensions of mineral materials, and, in particular, the use of such grinding agents to produce low viscosity aqueous suspensions of mineral solids. The grinding agents are described to be acrylic acid polymers and/or copolymers that are completely neutralised by at least one neutralising agent having a monovalent function and at least one neutralising agent having a polyvalent function.
EP-A2-0705892 teaches dispersions or slurries of solid particles in water which also comprise a phosphonocarboxylic acid or a water soluble salt thereof of the following formula, as a dispersing:
xe2x80x83H[CHRCHR]nxe2x80x94PO3M2/v
wherein at least one R group in each unit is a COOM1/v group and the other R group is hydrogen or a COOM1/v hydroxyl, phosphono, sulphono, sulphato, C1-7 alkyl group or a carboxylate, phosphono, sulphono, sulphato and/or hydroxy substituted C1-7 alkyl or C1-7 alkenyl group, and each M is a cation such that the phosphonated carboxylate is water soluble and n (the number of monomer units in polymerised form in the polymer chain) is from 1 to 6 and preferably less that 5, for example 1 to 3. The solid particles dispersed in this prior art preferably have a particle size of less than 1 mm, for example less than 0.5 mm, preferably 0.5 to 100 xcexcm especially 1 to 50 xcexcm.
The present invention seeks to provide improved dispersions or slurries of solid particles in water. It is well known, for example see PCT/US 91/94539, that it is especially difficult to achieve stable dispersions of fine materials; thus, it is of particular interest to produce stable dispersions of fine solid particles, such as those which have a significant proportion with a particle size of less than 2 xcexcm.
The present invention, therefore, provides a dispersion of particulate solid in a medium containing water and at least one dispersing agent, characterised in that said dispersing agent comprises a water soluble polymer comprising: 
wherein X is H, Na, K, Ca, Mg, NH4+ or A; and A is a polymer, copolymer or water soluble salt thereof, comprising one or more of the following monomers in polymerised form: 
wherein R1 is H, OH, C1 to C9 alkyl or alkoxy or acetoxy or acetate
R2 is H, C1 to C3 alkyl or alkxy, COOR3 
R3is H, Na, K, or C1 to C10 alkyl;
hydroxylpropyl acrylate, propyl methacrylate, 2-acrylamido-2-propane sulphonic acid, sodium styrene sulphonate, sodium allylsulphonate, sodium methyl sulphonate, vinyl sulphonic acid, and salts thereof; acrylamide, methacrylamide, tert-butylacrylamide, (meth)acrylonitrile, styrene, vinyl acetate allyloxy-2-hydroxypropyl sulphonate and dialkylacrylamide; and further wherein A contains at least 10 monomers in polymerised form.
The number of monomers in polymerised form can be from 10 to more than 100, but preferably it is from 15 to 85 and most preferably from 25 to 60. The dispersing agents may have a weight average molecular weight of from 1000 to 10,000, and dispersing agents having a weight average molecular weight of from 2000 to 6000 are especially preferred. Although stable dispersions with a low viscosity can be obtained with dispersing agents of a higher molecular weight, there is an increased tendency for gel formation with polymers of molecular weight above 6000 which may be disadvantageous in some applications.
The invention is particularly suited to forming dispersions of very fine particles, for example, those in which 70% or more of the particles are below 2 xcexcm, and preferably those in which 70% or more are below 1 xcexcm. The invention works particularly well with dispersions of fine particles in which 95% or more of the particles are below 1 xcexcm.
The water soluble dispersing agent is preferably a phosphono terminated homo- or co-polymer of acrylic acid. The other monomer if the dispersing agent is a copolymer may be maleic acid or maleic anhydride, methacrylic acid, sulfonic unsaturated monomers or any unsaturated polymerisable monomers having a COOH group. Such polymers may be made, for example, according to U.S. Pat. No. 4,046,707, U.S. Pat. No. 5,376,731, U.S. Pat. No. 5,077,361 and U.S. Pat. No. 5,294,686.
The dispersed or suspended solid may be any particulate solid which is sufficiently chemically inert to be dispersed or suspended in an aqueous medium. The particles are preferably inorganic, and substantially insoluble in water. Examples include calcium carbonate, clays, kaolin, talc and metallic oxides.
The liquid medium may be water, or a water-containing solution. This medium may also contain alcohols, glycols, surfactants or wetting agents.
The composition of the present invention provides at least two important advantages, namely, a reduction in viscosity of the dispersion and a reduction in the tendency of the dispersion to form a gel particularly in fine particle dispersions. In addition, these benefits are obtained for systems in which the percentage by weight of the solid particles in the dispersion is high, for example greater than 60% and preferably greater than 75%.
Finally, the invention also provides a method of reducing the viscosity of a dispersion of solid particulate material in an aqueous medium, comprising contacting the aqueous medium, either before or after forming the dispersion of the solid particulate material, with an effective amount of at least one dispersing agent, wherein said dispersing agent comprises a water soluble polymer comprising: 
wherein X is H, Na, K, Ca, Mg, NH4+ or A; and A is a polymer, copolymer or water soluble salt thereof, comprising one or more of the following monomers in polymerised form: 
wherein R1 is H, OH, C1 to C9 alkyl or alkoxy or acetoxy or acetate
R2 is H, C1 to C3 alkyl or alkxy, COOR3 
R3 is H, Na, K, or C1 to C10 alkyl;
hydroxylpropyl acrylate, propyl methacrylate, 2-acrylamido-2-propane sulphonic acid, sodium styrene sulphonate, sodium allylsulphonate, sodium methyl sulphonate, vinyl sulphonic acid, and salts thereof; acrylamide, methacrylamide, tert-butylacrylamide, (meth)acrylonitrile, styrene, vinyl acetate allyloxy-2-hydroxypropyl sulphonate and dialkylacrylamide; and further wherein A contains at least 10 monomers in polymerised form.
There are three general methods for making a dispersion of calcium carbonate fines:
1) combine natural calcium carbonate (including marble dust), water and dispersing agent together and mix to form a slurry;
2) precipitate calcium carbonate from an aqueous solution of soda lime (calcium hydroxide) and carbon dioxide, collect the precipitated material and mix with water and a dispersing agent to form a slurry; and
3) grind calcium carbonate to the desired particle size and mix with water and a dispersing agent to form a slurry.